Method of nitriding steel



United States Patent 3,232,797 METHOD OF NITRIDING STEEL Irwin I. Bessen, Monroeville, Pa., assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., 21 corporation of Pennsylvania Continuation of application Ser. No. 201,194, June 8, 1962. This application Feb. 28, 1964, Ser. No. 349,791 6 Claims. (Cl. 14816.6)

This application is a continuation of my application Serial No. 201,194, filed June 8, 1962, now abandoned.

This invention relates to a method of nitriding steel. It is more particularly concerned with a method of nitriding thin steel in the form of a coil of strip or of wire, or a pack of sheets.

Inexpensive thin steel sheet and strip of high strength and good ductility are desirable articles of commerce, particularly as a base for tinplate. Thin steel in the form of wire also has many uses. Application Serial No.

116,300, now Patent No. 3,139,359, of Eric R. Morgan, t

which is assigned to the assignee of this application, deals with a method of manufacturing thin steel of high strength and good ductility. One embodiment of the invention of that application consists of reducing the steel to an intermediate gauge by conventional hot and cold reduction steps, annealing the steel in a non-oxidizing atmosphere containing anhydrous ammonia so as to nitride the steel, and then further cold-reducing the nitrided steel to workharden and strengthen it. In accordance with that application, the steel must be disposed during the nitriding step so that the nitriding atmosphere has access to substantially the entire steel surface. This, can be accomplished if the steel is nitrided in the form of a strand or in the form of a loosely-wound coil. It can also be accomplished if the steel is nitrided in the form of a pack of sheets spaced one from another. Many steel plants, however, do not have the rather expensive facilities required for strand heat-treating. Furthermore, the provision of steel in the form of loosely-wound coils or in packs of separated sheets is expensive either in terms of the facilities or the labor required.

It is an object, therefore, of my invention to provide a processof nitriding thin steel which requires no special furnace and no special coiling or pack-forming apparatus. It is another object of my invention to provide a process of nitriding thin steel in a nitrogen-containing atmosphere generated at the surface of the steel. Other objects of my invention will appear in the following description thereof.

I have found that thin steel in coil form or in the form of stacked sheets can easily be nitrided in conventional heat-treating furnaces if the steel is first coated with a film of a compound which will release nascent nitrogen in a controlled temperature range and is then heated to that temperature range. I have discovered that guanidine compounds are suitable nitrogen-releasing compounds for my purposes. I have also found that films of suitable compounds can be deposited on the steel from water solutions of those compounds.

.In an embodiment of my invention presently preferred by me, I prepare conventional low-carbon steel strip for nitriding by passing the strip in the form of strand through an aqueoussolution of a guanidine compound. Excess solution is allowed to drip off the steel, and that remaining on the steel is dried thereon. The coated steel is tightly coiled in the conventional way and the coils are heated under covers to a temperature at or slightly above that at which the guanidine compound decomposes. The steel is held at that temperature for about a half an hour, or longer if desired, and then allowed to cool. The covers are sealed in the usual way to prevent access of 3,232,7 97 Patented Feb. 1, 1966 atmospheric oxygen to the steel during the heating and cooling cycle. The guanidine compounds have the virtue of resisting decomposition at low temperatures and of releasing substantial proportions of their nitrogen contents in the temperatur range most effective for nitriding. That range, I have found, is above about 600 F. Steel of tinplate gauge-about .010" in thickness-treated as above described, exhibited nitrogen contents as high as 080% and the nitriding extended through the entire thickness of the steel. Those nitrided steels, after temper rolling and strain aging, have tensile strengths in the range from 72,000 to 90,000 psi. Those values are far above the usual level of 50,000 to 58,000 psi. for conventional low-carbon steels of the same gauge.

In my process the film of nitrogen-releasing compound deposited on the surface of the steel decomposes at the temperatures reached in the heating furnace to release nascent nitrogen in direct contact with every portion of the surface of the steel, so that the steel is nitrided deeply and uniformly. This contact between nitrogen and steel does not require access of the atmosphere surrounding the coil to the coil interior and is independent of the tightness or looseness with which th strip is coiled. Because of this, my process is as well suited for stacked sheets as it is for coiled strip and it is equally suitable for coiled wire. The steel to be treated by my process, after being provided with a coating or film as above described, may be formed into a furnace charge of any type.

Certain aspects of my invention will best be understood in connection with the attached figures to which reference is now made. FIGURES la, 1b, 1c and 1d illustrate thermal decomposition properties of four nitrogen-releasing compounds. Those of FIGURES 1a and lb are suitable for my process while those of FIGURES 1c and 1d are unsuitable for my process.

FIGURE 2 illustrates graphically the nitriding effect in my process of three nitrogen-releasing compounds, one of which is suitable and the other two of which are unsuitable for my process, as well as certain physical properties of the steels so treated and an untreated steel. In FIGURE 2 are identified the solid and broken lines there used to represent the various physical properties of the steels, and which are used in the same way in FIGURES 3 and 4 to be mentioned.

FIGURE 3 illustrates graphically the effect of the thickness of the film of nitrogen-releasing compound on the properties of steels treated my my process.

FIGURE 4 illustrates graphically the effect of my process as applied to flat steels of three different gauges.

As I have mentioned, guanidine compounds are nitrogen-releasing compounds which I prefer for my process. The bar charts of FIGURES 1a and 111 show how guanidine carbonate and cyanoguanidine, respectively, decompose as they are heated to release nitrogen. In each case, and also in the case of the compounds of FIGURES 1c and 1d to be mentioned, the nitrogen released was reacted with .5 normal sulphuric acid to form ammonium sulphate and the amount of acid so consumed, in milliliters, is the ordinate of the chart. Guanidine carbonate begins to release nitrogen in the 300400 F. temperature range and releases appreciable amounts below 600 F. However, the temperature range of maximum nitrogen release is 600 to 800 F. and substantial amounts are released up to 1300 F. Cyanoguanidine releases no nitrogen below about 500 F. and releases the bulk of its nitrogen between 600 F. and 1200 F.

FIGURES 1c and 1d show why neither ammonium carbonate nor urea is suitable for my process. The salt first mentioned releases all of its ammonia below 500 F.a temperature too low for effective nitriding of steel to take place. Urea is little better, as it releases only insignificant amounts of ammonia above about 700 F.

In FIGURE 2 is shown the nitriding effect in my process of guanidine carbonate on a number of samples of lowcarbon steel. Both the nitrogen content and the physical properties of the nitrided samples are plotted as bars. The physical properties plotted, in order of increasing magnitude, are the yield strength of the steel, its tensile strength before strain-aging, and its tensile strength after strain-aging. For comparison the nitrogen content and the same physical properties are plotted for a number of steel samples similarly processed, but with films of the compounds ammonium carbonate and ammonium chloride respectively, each of which is unsuitable for my process. Likewise are plotted the nitrogen content and the same physical properties for a number of control samples which were heat treated in the same way as the other samples, but which were provided with no film or coating. Each sample indicated was heat-treated at 1300 F. for one hour in a non-oxidizing atmosphere.

The steels coated with the guanidine carbonate solu tion are seen to have been nitrided to a nitrogen content of more than 0.08% whereas the steel samples coated with the other nitrogen-releasing compounds displayed no increase in nitrogen content over the control samples. The steels coated with the guanidine carbonate solution exhibited yield Strengths of 50,00057,000 p.s.i., which is a substantial increase over the 42,00047,000 p.s.i. yield strengths of the control samples. The steels coated with the other nitrogen-releasing showed no improvement in yield strengths over the control samples. The guanidine carbonate treated steels had tensile strengths before strainaging of 65,000-70,000 p.s.i., while the control samples had tenisle strengths ranging from 52,00 to 56,000 p.s.i. The samples treated with ammonium carbonate and those treated with ammonium chloride had tensile strengths, before strain-aging, below those of the control samples, and even after strain-aging, displayed tensile strengths below the tensile strengths before strain-aging of the guanidine carbonate treated samples.

In FIGURE 3 are plotted the physical properties previously discussed for two sets of fiat samplesone set dipped in an aqueous solution of a guanidine compound and allowed to drip dry and the other passed through a wringer after dipping. Both sets were then heated at 1300 F. for one hour in a non-oxidizing atmosphere. The yield strengths and the tensile strengths both before and after strain-aging are all substantially higher for the samples with the heavier coating of nitrogen-releasing compound.

In FIGURE 4 are plotted the physical properties previously discussed for three sets of samples treated by the same process of my invention as is described in the next preceding paragraph, except that the coating applied to all samples of the same amount. The three sets comprised fiat samples .008", .0105" and .0125" thick respectively. The physical properties after nitriding are highest for the thinnest samples, which indicated that the thinner sheets were more completely nitrided than the thicker sheets under the conditions of the experiment. Thicker sheets thus require thicker films of nitrogenreleasing compound for maximum enhancement of their properties by the process of my invention.

In my process the edges of coiled strip or stacked sheets and the outside wraps or sheets are necessarily exposed to the furnace atmosphere as well as to the atmosphere resulting from the decomposition of the nitrogen-releasing compound. The nitrogen concentration in those regions thus is diluted by the atmosphere of the furnace. The nitriding reaction proceeds at a rate determined by the partial pressure of any nitrogen in this atmosphere and if that partial pressure falls to a low enough value, the reaction will reverse and nitrogen from the edges and exposed surfaces of the nitrided steel will pass into the furnace atmosphere. To avoid this denitridfour-hour soak at a maximum temperature.

4 ing of exposed surfaces of the steel, I introduce a small quantity of externally generated ammonia into the furnace atmosphere. The quantity required is that sufiicient to maintain equilibrium between the nitrogen in the furnace atmosphere and that in the steel.

My process is well adapted to the treatment of tin plate grade steel. It is advantageous to use steel containing as high a nitrogen content as can conveniently be achieved by normal steel making practices, as the quantity of nitriding compound required by my process is thereby reduced. Nitrogen contents up to about 015% in the steel to be treated are not difiicult to obtain, and are preferred by me. As a specific example of my invention I treated steel having the following ingot analyses:

The steel was hot-rolled to .080" thickness, pickled, and cold-rolled on a tandem mill to .0123" thickness. After being cold-rolled, the strip was cleaned by conventional methods. The clean strip was then coated with a saturated aqueous solution of guanidine chloride at a temperature of F. by allowing the solution to flow from a raised container onto the strip. The strip was immediately passed through a pair of rolls which were adjusted to produce a spreading but not a severe wringing effect. The coated strip, after passing through the rolls, was coiled and allowed to stand until it reached room temperature. It was then heated to a temperature of 1200 to 1225 F. in a twenty-hour cycle which included a A nonoxidizing atmosphere containing 0.5% ammonia was maintained in the furnace to minimize denitriding of the coil edges and outer wraps. The coil coated as above described was heat-treated along with an uncoated coil of the same material as a control.

After being heat-treated, the coils were given a temper pass of 1 /2 to 3% reduction. They were electrolytically tinned with a coating weight of .25 pound of tin per base box. The physical properties of the coils are summarized in Table I.

It will be observed from the table that the coated steel has a nitrogen content ranging from .028.05l%. It will also be noted that the control steel analyzed less than 009% nitrogen after heat treating which indicated that insufiicient free ammonia was present in the furnace atmosphere to prevent denitriding during the heating. In other examples, I have raised the ammonia content of the furnace atmosphere to 2% and find no denitriding effect on the controlled steel. From Table I it can be seen that steel treated in accordance with my invention exhibits an increase in yield strength of about 8,000 p.s.i. and an increase in tensile strength of about 15,000 psi. After strain aging the strength is additionally increased between 4,000 and 8,000 psi. Accompanying the increase in strength is a reduction in ductility as measured by the percent elongation in two inches. This reduction amounts to about 8% for steel in the unaged condition, leaving the steel still sufiiciently ductile for most fabrication purposes.

I find that it is satisfactory to apply the aqueous solution of guanidine compound by spraying, rather than by flowing, and that a satisfactory guanidine chloride solution for spraying contains 20 pounds of the compound per gallon of water.

The heat treating step of my process herein described may be effected by the annealing treatment conventionally given to coldreduced low-carbon steel intended for tinplate applications.

I claim:

1. The method of treating low carbon steel stock of thin cross section capable of being formed into a charge in which successive layers of the stock are in close abutting relationship, comprising coating the surface of the steel stock with a film of a nitrogen-releasing compound which decomposes and releases the major portion of its nitrogen at a temperature above about 600 F., forming the stock into a charge of successive layers having their adjacent surfaces in close abutting relationship with each other, and heating the charge to a temperature above about 600 F. at which the nitrogen-releasing compound is substantially completely decomposed for at least a half an hour in an atmosphere consisting essentially of that re sulting from the decomposition of the nitrogen-releasing compound, so as to nitride the steel.

2. The method of claim 1 in which the nitrogen-re leasing compound decomposes and releases a substantial portion of its nitrogen at a temperature below about 1300? F.

3. The method of treating thin low carbon steel comprising coating the surface of the steel with a film of a guanidine compound, forming the steel into a charge in which adjacent surfaces of the coated steel are in contact with each other and heating the charge to a temperature at which the guanidine compound is substantially completely decomposed for at least a half an hour in an atmosphere consisting essentially of that resulting from the decomposition of the guanidine compound so as to nitride the steel.

4. The method of claim 3 in which the guanidine compound is cyanoguanidine.

5. The method of claim 3 in which the guanidine compound is guanidine carbonate.

6. The method of claim 3 in which the guanidine compound is guanidine chloride.

References Cited by the Examiner UNITED STATES PATENTS 3/1933 Sutton et al. 148-16.6 1/1953 Darby 148-16.6

OTHER REFERENCES DAVID L. RECK, Primary Examiner. 

1. THE METHOD OF TREATING LOW CARBON STEEL STOCK OF THIN CROSS SECTION CAPABLE OF BEING FORMED INTO A CHARGE IN WHICH SUCCESSIVE LAYERS OF THE STOCK ARE IN CLOSE ABUTTING RELATIONSHIP, COMPRISING COATING THE SURFACE OF THE STEEL STOCK WITH A FILM OF A NITROGEN-RELEASING COMPOUND WHICH DECOMPOSES AND RELEASES THE MAJOR PORTION OF ITS NITROGEN AT A TEMPERATURE ABOVE ABOUT 600*F., FORMING THE STOCK INTO A CHARGE OF SUCCESSIVE LAYERS HAVING THEIR ADJACENT SURACES IN CLOSE ABUTTING RELATIONSHIP WITH EACH OTHER, AND HEATING THE CHARGE TO A TEMPERATURE ABOVE ABOUT 600*F. AT WHICH THE NITROGEN-RELEASING COMPOUND IS SUBSTANTIALLY COMPLETELY DECOMPOSED FOR AT LEAST A HALF AN HOUR IN AN ATMOSPHERE CONSISTING ESSENTIALLY OF THAT RESULTING FROM THE DECOMPOSITION OF THE NITROGEN-RELEASING COMPOUND, SO AS TO NITRIDE THE STEEL. 